AP TOPIC 8: Chemical Bonding

Transcription

1 AP TOPIC 8: Chemical Bonding Introduction In the study of bonding we will consider several different types of chemical bond and some of the theories associated with them. TYPES OF BONDING INTRA (Within (inside) compounds) STRONG INTER (Interactions between the molecules of a compound) WEAK Ionic (Metal + Non-metal) Giant lattice of ions Covalent (Non-metals) Discrete molecules Hydrogen Bonding (H attached to N, O or F) Strong, permanent dipole Dipole-Dipole (Polar molecules) Permanent dipoles London Dispersion Forces (Non-polar molecules) Induced dipoles Dative or (Co-ordinate) (Electron deficient species) Discrete molecules To help distinguish the difference in strength of intra and inter bonds consider the process of boiling of water. When water boils the product is steam (gaseous water). The products are not hydrogen and oxygen. This is because the weak inter molecular forces are broken not the much stronger intra molecular forces. C:\ Page 1 of 20

2 Intra Bonding Ionic (the transfer of electrons between atoms to form ions that form giant ionic lattices) Atoms have equal numbers of protons and electrons and consequently have no overall charge. When atoms lose or gain electrons, (in order to achieve full outer shells or a noble gas structure and stability), the proton/electron numbers are unbalanced causing the particles to become charged. These charged particles are called IONS. Since metals have a tendency to lose electrons to form positive ions and non-metals the opposite, the ionic bond is usually formed between metals & non-metals. The strong electrostatic forces between the charged particles are called ionic bonds. Since the electrostatic forces are large the bond is a strong one. Task 8a List the ions present and hence the formula of the following compounds; Sodium chloride, calcium chloride, iron(iii) bromide, sodium oxide. Covalent (the sharing of electrons between atoms to form discrete molecules) In covalent bonding atoms once again want to achieve full outside shells, this time by joining together and sharing valence shell electrons. When they do so they form MOLECULES. One shared pair of electrons represents one covalent bond, two shared pairs two bonds etc. These bonds usually occur between two atoms that are non-metals. Any electron pairs that occur in the outside shell of an atom but are not involved in bonding are called non-bonding electrons or lone pairs of electrons. Lewis structures of Covalently Bonded Molecules Lewis structures use dots to represent outer shell electrons in atoms when they form molecules. As discussed above, when atoms form molecules they share electrons to achieve full outer shells. In the case of hydrogen this is two (a duet), in the case of the second period non-metal elements this is eight (octet rule). Some elements in the third period can have more than eight electrons in their outer shells (expanded octet rule), but very often atoms are trying to achieve a full octet of valence electrons. Drawing Lewis structures 1. Calculate the total number of valence shell electrons (taking into account any charges present. Adding electrons for negative charges, and subtracting electrons for positive charges). 2. In a species with more than two atoms, decide which atom is the central one (this is usually obvious, but if in doubt it will be the least electronegative atom but never hydrogen). Use one pair of electrons to form a covalent bond between terminal (outer) atoms that are bonded to the central atom. 3. Arrange the remaining electrons to complete the octets of the terminal atoms and then place any remaining electrons on the central atom, if necessary expanding the central atom s octet. 4. If the central atom lacks an octet, form multiple bonds (double or triple) by converting nonbonding electrons from terminal atoms into bonding pairs. (Some atoms are exceptions and remain electron deficient). 5. One bonding pair of electrons represents one covalent bond that in turn can be represented by one line ( ). Task 8b Draw Lewis diagrams for each of these species; F 2, O 2, N 2, HCl, HF, H 2 O, NH 3, CBr 4, PF 5, PCl 6 - and NH 4 +. C:\ Page 2 of 20

3 Shapes of covalently bonded molecules and ions The shapes of covalently bonded molecules and ions can be determined by considering the number of electron pairs around the central atom. The electron pairs repel one another and try to get as far apart as possible. This theory is called Valence Shell Electron Pair Repulsion theory or VSEPR. There are some standard shapes for specific numbers of electron pairs and some simple deviations from this theory when non-bonding (lone) pairs are present around the central atom. A non-bonding pair will repel more strongly than a bonding pair. When comparing bond angles it can be seen that this has the effect of altering the bond angles and the shape of molecules that have a similar total number of electron pairs around the central atom, but where the total is made up of different combinations of bonding and lone pairs. N.B. For the purposes of predicting shape multiple bonds can be considered as single bonds (or single negative centers). E.g. A double bond is considered to be just one bonding pair e.g. carbon dioxide can be considered to be surrounded by two electron pairs (negative centers) and is therefore linear. Predicting shapes of molecules 1. Draw the Lewis structure for the molecule. 2. Count the electron pairs around the central atom and arrange them in a way that minimizes repulsion (i.e. keep them as far apart as possible). 3. Use the table below to recall the correct shape, name and bond angles that correspond to the position of the atoms in the structure. Task 8c Draw Lewis structures and sketch the molecular shapes for the following species. In each case identify the numbers of bonding pairs and lone pair around the central atom, and predict the bond angles. PCl 6 -, ICl 3, BrF 5, SO 3 2-, CH 4, NH 4 +, ICl 4 - and SO 2. C:\ Page 3 of 20

4 Number of electron pairs around central atom o Geometry of BONDING (B) LONE (E) Example Bond angles Electron Pairs Full description of the molecule Geometry of Atoms 3D Shape Type (A is central) 2 0 BeCl Linear Linear AB BF Trigonal planar Trigonal Planar AB SO 2 Slightly less than 120 Trigonal planar Bent or V Shaped AB 2 E 4 0 CH Tetrahedral Tetrahedral AB NH Tetrahedral 2 2 H 2 O Tetrahedral Trigonal Pyramidal Bent or V Shaped AB 3 E AB 2 E PCl in plane. 90 perpendicular to plane Trigonal bipyramidal Trigonal Bipyramidal AB SF 4 Complex Trigonal bipyramid Seesaw AB 4 E 3 2 ClF 3 Approx. 90 Trigonal bipyramidal T-Shaped AB 3 E XeF Trigonal bipyramid Linear AB 2 E SF 6 90 Octahedral Octahedral AB BrF 5 Approx. 90 Octahedral Square Pyramidal AB 5 E 4 2 XeF 4 90 Octahedral Square Planar AB 4 E 2 C:\ Page 4 of 20

5 Resonance structures & Bond length Occasionally when drawing a Lewis structure that involves multiple bonds, it may be possible to draw a number of different, feasible, Lewis structures. In such a case we call the various structures resonance structures. E.g. Carbon dioxide has three possible Lewis structures. The carbonate ion provides another example. The Lewis structure for the carbonate ion can be represented by the diagram below. However, it would be equally correct to have created the double bond on the right hand oxygen or the left hand oxygen rather than the oxygen at the top of the diagram and hence get a total of three different resonance structures. The reality is that the structure is an average of all three possible resonance structures and each bond is really one and one third covalent bonds rather than any combination of two singles and one double. If a multiple bond (double or triple) is created between two atoms the bond length observed will be shorter than the corresponding single bond. This is because a double bond is stronger than a single bond and hence pulls the atoms closer together. A triple bond is correspondingly shorter and stronger than a double bond. Evidence for the one and one third bond order in the carbonate ion is that all the bonds are found to be the same length, not the different lengths that one would expect to find if a combination of double and single bonds were present. Formal Charge The formal charge of an atom within a Lewis structure is generally used in one of two ways. (i) To suggest where charges may most reasonably lay (for example where the 2- charge of the carbonate ion actually resides) First, calculate the formal charge on an atom within a Lewis structure, apply the following formula. The number of The number of The number of Formal Charge on an atom valence electrons non bonding electrons 1 bonding electrons within a Lewis structure around that atom around that atom 2 around that atom in the free atom in Lewis structure in Lewis structure C:\ Page 5 of 20

6 Then, using the Lewis structure for the carbonate ion (CO 3 2- ) as an example It is possible to determine the formal charges on the atoms and therefore the most likely separation of charge. Carbon atom Formal charge = 4 0 ½ (8) = 0 Oxygen atom in C=O Formal charge = 6 4 ½ (4) = 0 Each oxygen atom in C-O Formal charge = 6 6 ½ (2) = -1 Note that the sum of the formal charges adds up to the total charge on the ion or molecule. (ii) To help select the most plausible structure from a set of resonance structures Again, calculate the formal charge on an atom within a Lewis structure, apply the following formula. Using the two possible Lewis structures for CH 2 O with formal charges added as an example, it is possible to determine the most likely Lewis structure for the compound. Again, the sum of the formal charges adds up to the total charge on the species. In order to determine which structure is most likely, choose the structure with zero formal charges, and/or formal charges with absolute values as low as possible, and/or keep any negative formal charges on the most electronegative atoms. Since the first structure has no formal charges it is considered to be the most plausible. C:\ Page 6 of 20

7 Dative or Co-ordinate Bonding (Electron deficient species) Occasionally when drawing a Lewis structure you may encounter a molecule with a central atom that does not have a complete octet of electrons surrounding it. One such example is BF 3. In this molecule the Boron atom has only six electrons surrounding it rather than the required eight. The Boron atom is said to be electron deficient. It can make up the octet by forming bonds with other compounds that have non-bonding pairs of electrons, e.g. ammonia (NH 3 ). The new, shared pair (covalent bond) formed is made up by using both the electrons from one species (in this case the nitrogen atom in the ammonia molecule) rather than one from each species as in a normal covalent bond. This type of covalent bond is called a dative or coordinate bond. C:\ Page 7 of 20

8 Electron clouds and the sliding scale of bonding type An electron cloud diagram can be thought of as a probability map of where an electron may be found at any one time. In separate atoms this is very simple. When a chemical bond forms there is a redistribution of the outer electron clouds. Electrons may be completely transferred (100% ionic bonding) Or completely equally shared (100% covalent, non-polar covalent bonding) In the simplest terms it is possible to think of compounds being either 100% ionic or 100% covalent but actually it is necessary to consider these two situations being at the extremities of a sliding scale. In fact, most bonds are intermediate between the two, with a largely covalent substance actually having a degree of ionic behavior, and vice-versa. Ionic substance with some covalent character If the cation is small and highly charged it will have the ability to distort the charge cloud around the anion thus; Comparing this diagram with the charge cloud representations of 100% ionic bonding and 100% covalent bonding it is possible to observe the ionic bond beginning to acquire some covalent character. Fajans rules help to assess the degree of distortion (POLARISATION). Distortion will be at a maximum when; 1. The cation is small and highly charged, i.e. has a high charge density. 2. When the anion is large and highly charged, i.e. electrons are more loosely held. C:\ Page 8 of 20

9 Covalent substance with some ionic character (Polar Covalent) Electronegativity is the ability of an atom within a covalent bond to attract electrons to itself. When, in a covalent bond, one atom has a higher electronegativity than the other, the electrons are attracted toward that atom leading to a charge cloud distortion thus; Comparing this diagram with the first two charge cloud representations, it is possible to observe the covalent bond beginning to acquire some ionic character. Using the Pauling scale it is possible to make predictions about the degree of ionic and covalent character in a compound. By considering differences in electronegativity and by using the table below, one can predict the % of covalent and ionic character present in a bond between two atoms. Electronegativity difference between the atoms in the covalent bond Approximate Percentage of ionic character A difference of approximately 1.7 marks the boundary between predominately ionic or covalent character C:\ Page 9 of 20

10 Polar bonds and Polar molecules In order for a substance to be polar, bonds within the molecule must carry different charges (i.e. a dipole moment must exist) AND the dipoles that are present must NOT cancel out due to symmetry. The dipole moment can be indicated by an arrow that points toward the negative charge center with the tail of the arrow indicating the positive charge center, or by using + and - to indicate small areas of positive and negative charge. For example, consider 2-pentanone and trifluoromethane are both polar. Like water, (which is also polar), opposite ends of the molecules carry different charges and there is no canceling of the dipoles. Polar 2-pentanone Polar trifluoromethane Now consider hexane and carbon tetrachloride, neither are polar molecules but for different reasons. In Hexane (C 6 H 14 ), carbon and hydrogen have very similar electronegativities and as a result the bonds are effectively non-polar. Carbon tetrachloride (CCl 4 ) on the other hand has four polar C-Cl bonds but because of its symmetrical shape is non-polar overall. The partial positive charge is located at the center of the molecule and the partial negative charges equally spread around it, causing the dipoles to cancel. Examples of other compounds that are non-polar as a result of symmetry and the cancellation of dipoles are CO 2 and BCl 3. Non-Polar carbon dioxide Non-Polar Molecule boron trichloride C:\ Page 10 of 20

11 Inter Bonding Hydrogen Bonding Hydrogen is an exceptional element in that when it forms a covalent bond its electron is held to one side of the nucleus leaving the other side completely exposed. Any approaching negatively charged group can get very close to the hydrogen nucleus and this produces an unexpectedly large electrostatic attraction. These electrostatic attractions are exaggerated when H is bonded to a more electronegative element e.g. F, O or N. Such electrostatic attractions are called hydrogen bonds. The occurrence of hydrogen bonds has two important consequences. (i) (ii) It gives substances containing them anomalously high boiling points Substances containing them tend to be more viscous. Both are explained by the increased attraction between molecules caused by hydrogen bonding making it more difficult to separate them. For example, in water, black dots represent oxygen atoms and white dots represent hydrogen atoms. C:\ Page 11 of 20

12 Dipole-Dipole interactions When molecules that have permanent dipoles come together, they will arrange themselves so that the negative and the positive ends of the molecules attract one another. The attractions are called dipole-dipole interactions. The molecules eventually align in order to find the best compromise between attraction and repulsion. London Dispersion Forces These are small electrostatic forces that are caused by movement of electrons within the covalent bonds of molecules that would otherwise have no permanent dipole. As one molecule approaches another the electrons of one or both are temporarily displaced owing to their mutual repulsion. This movement causes small, temporary dipoles to be set up which attract one another. These attractions are called London Dispersion Forces. These dispersion forces increase with the size of the molecule and with its surface area. Large molecules with big surface areas have more electrons, and more dispersion forces and greater attraction and therefore higher boiling points. C:\ Page 12 of 20

13 Other aspects of bonding theory The solid state Variations in bond type lead to different structures in solids. Four different types of solid structure and their properties are considered here. A: Giant atomic structures. E.g. Diamond and graphite. These structures are made of carbon atoms covalently bonded together in a continuous network. The diamond structure is based upon the tetrahedral unit where all of the carbon atoms are bonded to one another with very strong covalent bonds in a huge macro structure. This makes diamond very strong and hard. Graphite has a layered structure where each carbon atom is bonded to three others in each plane. The structure leads to specific properties; (i) It will conduct electricity only in one plane In the graphite structure each carbon is only bonded to three others. This leaves one of the outer electrons "free". Each carbon atom has one such "free" electron and these are spread out over each layer (delocalized). This leads to a "sea" of electrons similar to that in the metallic structure and graphite's ability to conduct electricity along the layers. Since the electrons cannot move between layers there is no conduction in the vertical plane. (ii) It can be used as a lubricant Weak dispersion forces hold the layers in graphite together represented by the dotted lines on the diagram. As a result they can slide over one another, making graphite a good lubricant. C:\ Page 13 of 20

14 B: Ionic structures. E.g. Sodium chloride. In the ionic structure the lattice of ions is held together by strong electrostatic interactions between them. The strong bonds give ionic solids high melting points. They can only conduct electricity when molten or in solution since in the solid the ions are rigidly held and cannot move. Hydration of ions and the solubility of ionic compounds The charged particles present in an ionic solid explain how and why ionic solids dissolve in water. When an ionic solid dissolves the polar water molecules penetrate the lattice and attach themselves to the ions. The process is called hydration and the ions are said to be hydrated. C:\ Page 14 of 20

15 C: Molecular structures. E.g. Iodine Iodine is held in a similar lattice structure to NaCl but in this case only weak dispersion forces hold the I 2 molecules in place. This means that solid iodine has a very low melting point since there is only a weak interaction between its molecules. In addition there are no charged particles present making iodine a nonconductor. D: METALLIC BONDING (the structure of metals) A metals' structure can be considered to be a close packed lattice of positive ions surrounded by a "sea" of electrons. These electrons and their movement cause metals to be good conductors of electricity. The close packed ions make them good conductors of heat. The liquid state The metallic bond is the electrostatic attraction between the positive and negative charges. The flexibility of these bonds makes metals malleable & ductile. In a gas the molecules possess enough energy to overcome the intermolecular forces, and hence move around freely. When a gas is cooled the kinetic energy of its molecules falls and the intermolecular forces become increasingly important. Cooling eventually leads to them having insufficient energy to overcome these forces and the gas condenses to form a liquid. In a solid the particles have a regular ordered structure and move very little around fixed positions. As solids are heated the particles gain energy and can break away from this ordered arrangement forming a liquid. The liquid represents the molecules at an intermediate state between the ordered state of the solid and the disordered state of the gas. Vapor pressure Consider a liquid in a sealed container. Even if the liquid is below its boiling point a few of the molecules will possess enough energy to overcome the intermolecular forces holding them together and escape into the vapor phase above the liquid. The weaker the intermolecular forces between the molecules, the easier this process will be and the more molecules will enter the vapor phase. This causes a relatively high vapor pressure. So, in summary, weak intermolecular forces cause liquids to have low boiling points, they are said to be volatile and will have high vapor pressures, and vice-versa. C:\ Page 15 of 20

16 The gaseous state Boiling points of the Noble gases On descending group VIII the atomic number increases, the atoms get bigger, have more electrons and larger surface areas. This increases the London dispersion forces between them making them more difficult to separate and increasing their boiling point. Element He Ne Ar Kr Xe Boiling Point / o C Boiling points of the Hydrides of Groups IV, V, VI and VII Group IV Hydride CH 4 SiH 4 GeH 4 Boiling Point / o C The boiling point increases due to increased size and dispersion forces. Group V Hydride NH 3 PH 3 AsH 3 SbH 3 Boiling Point / o C A similar pattern (of increase of size, dispersion forces and boiling points) is observed here except for ammonia. The ammonia molecules are attracted to one another by hydrogen bonds giving ammonia a higher boiling point than would normally be expected. Group VI Hydride H 2 O H 2 S H 2 Se H 2 Te Boiling Point / o C Group VII Hydride HF HCl HBr HI Boiling Point / o C The patterns in groups VI and VII resemble that of group IV with water and hydrogen fluoride respectively exhibiting the higher boiling points associated with hydrogen bonding. Valence Bond theory Valence bond theory assumes that the electrons in a molecule occupy the atomic orbitals of the individual atoms. Sigma () and Pi () bonds When atoms form covalent bonds there is an overlap of the atomic orbitals to form the molecule. For example consider two hydrogen atoms forming H 2. Each hydrogen atom has one electron that occupies a spherical 1s orbital. On forming H 2 these 1s orbitals overlap to share a pair of electrons forming a molecule and a covalent bond. Now consider two oxygen atoms forming O 2. Each oxygen atom has the electronic configuration [He] 2s 2 2p 4. C:\ Page 16 of 20

17 The two, dumbbell shaped, 2p orbitals that are incomplete can take part in bonding and there is a simple end on end overlap that occurs. The electron cloud density is located along the axis of the bond and this is called a sigma () covalent bond. There is also scope for another type of bond that occurs from the sideways overlap of the other p orbitals. Here the electron cloud density is located above and below the axis of the bond this is called a pi () bond. As a result the oxygen atoms form a double bond giving the familiar oxygen molecule, O=O or O 2. Polyatomic molecules and hybridization of orbitals When considering a polyatomic molecule the question of shape must be accounted for. Valence bond theory incorporates shape by assuming the hybridization (mixing) of outer (valence) shell orbitals. Consider the example in the table below of the sp 3 hybridization of carbon in the methane (CH 4 ) molecule. In its ground state the carbon atom has two unpaired electrons and as such it may only make two bonds with hydrogen atoms to form the species CH 2. However this species is extremely unstable and does not exist. In order to form the much more stable CH 4 molecule (that does exist) one of the electrons in the 2s orbital is promoted into the empty 2pz orbital to form the excited state of [He] 2s 1 2px 1 2py 1 2pz 1. These four orbitals then mix together to form the four, sp 3 hybrid equivalent orbitals. These four hybrid orbitals then form simple sigma bonds with each of four hydrogen atoms to give four electrons pairs around the central carbon atom thus forming the familiar tetrahedral shape. In all of the examples below the terminal atoms join the central atoms hybridized orbitals using simple sigma bonds to form the final molecule. There are more complicated hybridization patterns involving pi bonds for polyatomic molecules that form multiple (double and triple bonds) that are not considered here. C:\ Page 17 of 20

18 Type of hybridization Example molecule Ground state of the atom undergoing orbital hybridization Number and type of hybrid orbitals Number and type of empty, un-hybridized orbitals Be sp BeCl2 Ground state would make 0 covalent bonds, hybridized can make 2 covalent bonds 2 2s 2px 2 2py 2pz B sp 2 BF3 Ground state would make 1 covalent bond, hybridized can make 3 covalent bonds 3 2s 2px 2py 1 2pz C 4 sp 3 CH4 Ground state would make 2 covalent bonds, hybridized can make 4 covalent bonds 2s 2px 2py 2pz 0 sp 3 d PCl5 P 5 Ground state would make 3 covalent bonds, hybridized can make 5 covalent bonds 3s 3px 3py 3pz 3d 4 3d 3d 3d 3d S 6 sp 3 d 2 SF6 Ground state would make 2 covalent bonds, hybridized can make 6 covalent bonds 3s 3px 3py 3pz 3d 3d 3 3d 3d 3d The type of hybridization present in a species is really quite simple to predict. By considering the total number of electron pairs (including both bonding and non-bonding pairs) around the central atom, one can determine the total number of orbitals that need to be present, since each electron pair needs one orbital. So, by taking (in order), one s, three p and as many d orbitals as required, one can determine the correct number of orbitals needed, and hence the hybridization. For example, six electron pairs require six orbitals and have a hybridization of (s + p + p + p + d + d) or sp 3 d 2. C:\ Page 18 of 20

19 Molecular orbital theory Valence bond theory is only one of two approaches used to explain bonding in molecules. Where valence bond theory fails to account for the observed behavior of molecules another more complex theory must be used. One such example is the unexpected paramagnetic behavior of oxygen. Its bonding can be explained using molecular orbital theory which describes covalent bonds in terms of the combination of individual atomic orbitals to form molecular orbitals rather than the independent overlap of the individual atomic orbitals. Molecular orbital theory is not discussed in any depth here. C:\ Page 19 of 20

20 Summary of Bonding related to properties Substances IONIC (GIANT lattice) DISCRETE COVALENT (INDIVIDUAL molecules) GIANT COVALENT (MACRO molecular) METALS/ALLOYS (MIXTURES of metals) +ve ion and ve ion formed via transfer of electrons held together in giant lattice with strong electrostatic interaction Small groups of atoms covalently bonded together by sharing electrons Massive structures of atoms covalently bonded together Close packed array of atoms (ions) with sea of free moving electrons Metal + Non-Metal e.g. KCl, MgF 2, Na 2SO 4 etc. Non-Metal + Non-Metal (compounds/polyatomic ions) e.g. CO 2, PCl 6 - (Lewis structures/shape) Elements and some compounds e.g. Graphite & diamond (carbon), Si, SiO 2 Metals e.g. Na, Al, Au, Stainless Steel (Fe/C/Cr), Bronze (Cu/Sn), Brass (Cu/Zn) STRONG bonds high m.p/b.p poor conductors of electricity when solid (ions NOT free to move), good when liquid or in solution (dissolved) will dissolve in polar solvents Strong covalent bond within the molecule, but molecules attracted to one another with WEAK IMF low m.p/b.p, often liquids or gases at RT; LDF (induced dipoles) Dipole (perm. dipoles) H-Bonds (H - N/O/F) poor conductors STRONG bonds high m.p/b.p hard poor conductors STRONG (but flexible) bonds high m.p/b.p good conductors of electricity ( sea of electrons) and heat (close packed) malleable (can be shaped), ductile (can be drawn into thin wires), luster (shiny), C:\ Page 20 of 20